Sleep Position/Sleep Alignment
Correct or incorrect alignment has a profound impact on alignment and physical pain. The healthiest sleep positions are (1) on your back with the legs straight but relaxed, arms down; (2) on your side with the knees together
There are three predominant misalignments regarding sleep position. Those are twisting the pelvic joints, twisting the shoulder joints, and twisting the joints of the spine (including the neck). The body tends to learn the misalignments which result at the deepest level, since we totally relax during our sleep and usually do not move for anywhere from several minutes to a couple hours!
Stomach sleeping is about as evil as it gets. Sleeping on one's stomach often involves lifting one leg, which twists one half of the pelvis backward. This is a prime cause of pain in the low back, SI joint, hip, as well as sciatic pain, knee and even foot pain. Stomach sleeping also twists the neck.
Frequent causes of pelvic misalignment and hip pain, sleeping on the side or sleeping on the back with one leg bent are probably the most common sleep misalignments.
The asymmetry of sleeping like this can also result in SI joint, low back pain and sciatic pain.
Sleeping with one or both arms above the head results in a lot of neck pain, as well as pain between the shoulder blades which usually occurs on one side. The rule of thumb is: keep the elbow below the shoulder. Raising the elbow above the level of the shoulder pulls on the shoulder blade, which in turn pulls on the rhomboids, which in turn pulls on the upper thoracic vertebrae and the ribs which attach in that area.
Side lying with one arm up: note elbow is above the shoulder
Supine with both arms up, note elbows above the shoulder.
It's fine to raise the elbows up, just keep them below the shoulder.
When sleeping on your side, your pillow should be high enough to support your head, so that your neck is in a straight line. If your pillow is too low, or too high, the neck will side flex and one-sided neck pain will eventually ensue.
Neck in a straight line
Pillow too low, neck side flexed to the left
Pillow too high, neck side flexed to the right
It's possible to unlearn or fix the bony misalignments and spasm which the body aquires when misaligned during sleep. However, it's not possible to relieve pain or heal many injuries unless sleep alignment is correct.